Paperboard feeders are well-known in the prior art and they include various types of feeder elements which drive the lowermost sheet of a stack past a gate to the nip rolls of a box-finishing machine. One type of feeder is a "kicker bar" which engages the trailing edge of the sheet and pushes it to the nip rolls. More recent feeders include segmented wheels which are shown in U.S. Pat. No. 4,045,015 and engage the underside of the sheet; whole wheels shown in U.S. Pat. No. 4,614,335 and U.S. patent application Ser. No. 06/674,294, filed Nov. 23, 1984, entitled "Rotary-Type Feeder Machines and Methods" and which also engage the underside of the sheet; and belts shown in U.S. Pat. No. 4,494,745. In these more recent feeders, a vacuum or suction is utilized to hold the sheet on the feed elements and some feeders also use a grate movable above and below the feed elements to establish or terminate driving engagement between the sheet and feed elements.
With all of these types of feeders of the prior art, once the sheet enters the nip rolls, the feed element is disengaged from the sheet leaving the nip rolls to continue the feeding of the sheet to the next station in the box-finishing machine. It is most important that the sheet be fed to the nip rolls in "register" and with "matched velocity", meaning that the velocity of the sheet must equal the surface velocity of tee nip rolls, and further that the nip rolls feed the sheet in synchronism with the moving parts of the box-finishing machine.
One of the problems attendant feeders of the prior art is that the weight of the sheet stack and the added pressure on the sheet produced by the vacuum, produces a drag on the sheet being fed resulting in loss of registry or control of the sheet. To compensate for the drag on the sheet, it is necessary to increase pressure on the sheet from the nip rolls by adjusting the spacing between the nip rolls. However this can result in crushing the paperboard sheet which, in turn, will weaken the sheet. It can also deform the surface of the nip rolls which may produce a velocity change, making it impossible to match the velocity of the sheet with that of the nip rolls, and the velocity of the nip rolls with that of the other parts of the box-finishing machine. Moreover, when feeding corrugated board having creases perpendicular to the direction of flow, control of the sheet may be lost when the crease enters the nip rolls due to the surface depression of the crease. In addition, increasing the pressure of the nip rolls accelerates the wear on the nip rolls as well as their bearings and gears, thus shortening the life of these parts and requiring repair and production downtime.